Nowadays telephone systems are not only used for transmitting conventional calls but also for offering a plurality of other services. New service concepts are designed constantly, in particular for radio telephone systems. These services are favoured by the users because most users always carry a mobile phone and thus the services are available all the time.
Different services require different transmission capacities from the radio connection. One of the most important research projects in the field of wireless telecommunication systems is how to increase the data transmission capacity over the radio connection. Various methods have been proposed to improve the capacity of existing radio systems and new systems as much as possible. However, each method has its own advantages and disadvantages.
An obvious method of increasing the data rate is to use a higher order modulation method. A disadvantage of such methods is, however, that in order to function properly they require a good signal-to-noise ratio. Secondly, in TDMA systems, in particular, the structure of the equalizer needed in the system becomes complex. The radio frequency parts of base stations and terminals typically generate non-linearity in a signal. Since interference is also generated in the signal, it is difficult to achieve a good signal-to-noise ratio.
Another alternative is to use diversity in signal transmission. Diversity allows improvement of the signal-to-noise ratio of a signal received in a receiver, which increases the average data rate. Prior art transmission diversity methods include delay diversity where the signal is transmitted twice, the latter transmission being delayed. This solution is, however, clearly suboptimal.
Diversity is achieved better by space-time block coding (STBC), which provides the full advantage of diversity. The space-time block code is described e.g. in Tarokh, V., Jafarkhani, H., Calderbank, A. R.: Space-Time Block Codes from Orthogonal Designs, IEEE Transactions on information theory, Vol. 45, pp. 1456–1467, July 1999, and in WO 99/14871, which are incorporated herein by reference.
The above-mentioned patent discloses a diversity method where the symbols to be transmitted, which consist of bits, are coded in blocks with a given length and each block is coded into a given number of channel symbols for transmission via two antennas. A different signal is transmitted via each antenna. For example, when the symbols to be coded are divided into blocks with a length of two symbols, the channel symbols to be transmitted are formed so that the channel symbols to be transmitted via a first antenna consist of the first symbol and the complex conjugate of the second symbol, and the channel symbols to be transmitted via the second antenna consists of the second symbol and the complex conjugate of the first symbol.
The STBC method functions well when there is only one antenna at the receiving end. If there are several antennas both at the transmitting and the receiving end, the STBC is suboptimal. As regards this, reference is made to S. Sandhu, A. Paulraj: Space Time Block Codes: A Capacity Perspective, IEEE Communications letters, Vol 4, No. 12, December 2000, which is incorporated herein by reference.
A yet another prior art method is to use several antennas or antenna arrays both in transmission and in reception. This is known as the MIMO method (Multiple Input Multiple Output). It has been suggested that the MIMO method yields better results than the methods described above. The MIMO is described in greater detail in G. J. Foschini, Layered Space-Time Architecture for Wireless Communication in a Fading Environment when Using Multi-Element Antennas, Bell Labs Technical Journal, Autumn 1996, which is incorporated herein by reference. A good capacity can be achieved by the MIMO, provided that the terminal of the radio system also comprises at least two antennas. Another disadvantage is that the MIMO functions well only if the signals transmitted and received via different antennas travel via different channels. This means that there should be hardly any correlation between the channels. If the channels correlate, the advantage obtained by the MIMO is minimal.